Excitation mechanisms of copper ionic and atomic lines emitted from a low-pressure argon laser-induced plasma

Low-pressure laser-induced plasmas generated with a pulsed Nd: YAG laser have complicated structures both temporally and spatially. The emission characteristics of the plasma are investigated for optimizing the experimental parameters in atomic emission spectrometry. The emission intensities of copper emission lines, measured in a time-resolved as well as a time-integrated mode, are strongly dependent on the kind of copper lines, ionic or atomic line, and the excitation energy. Also, the pressure of argon gas is the most important parameter for determining the behavior of these emission lines, including argon lines. Generally, copper ionic lines are dominantly emitted from the initial breakdown zone, because the copper ions are produced mainly in the hot breakdown zone. However, the Cu II 229.44-nm line is emitted also from the expansion zone of the plasma. It results from an additional excitation process through the charge-transfer collision particularly effective for the corresponding excited level. In this work, the excitation mechanisms for Cu I, Ar I, and Ar II lines are also discussed. The excitations occurring in the laser-induced plasma can be well understood by taking the temporal and spatial variations in their intensities into consideration.